This invention relates to signal coupling methods and arrangements which are particularly, though not exclusively, applicable to the coupling of signals to and from antennas.
One arrangement to be described below, by way of example in illustration of the invention, which is directed to minimising the effect of unwanted coupling between electrical circuits, has a four-port coupling network which has two main signal paths and which provides connection between two antennas and their respective associated equipment. The four-port coupling network also has an auxiliary path which provides a degree of coupling between one of the two main signal paths and the other. A characteristic of the cross-coupling is such that a proportion and quality of the signal in the one of the main signal paths is passed to the other path, according to the need to provide compensation for unwanted coupling between the antennas, and it can be adjusted to meet this need. Between the coupling network and each of the two antennas there is a respective antenna signal path and the electrical length of each of these paths may be so arranged that the signal which is deliberately cross-coupled between the signal paths is in anti-phase with an unwanted signal which has been transferred from one antenna to the other due to mutual coupling. The anti-phase property is provided, in general, by the appropriate choice of the lengths of the paths between the four port coupling network and each of the antennas. The proportion of the signal which is deliberately cross-coupled between the one main signal path and the other in order to effect the compensation is ideally selected or selectable to be of the same magnitude as the unwanted signal which has been derived from mutual coupling between the antennas, at the position at which the compensating cross-coupling occurs. In this way the effect of the unwanted coupling is minimised. There need not be a discrete or clearly defined device having four ports. It is possible to employ equipment which performs the same function. It is also possible to provide phase compensation by adjusting the phase of the deliberate cross coupled signal, either instead of, or in addition to the compensation provided by the lengths of the paths connecting the antennas to the coupling network.
A second arrangement to be described below, by way of example in illustration of the invention, is directed to the provision of a coupling which has a comparatively small profile, and which operates with a comparatively wide bandwidth with good performance, including at microwave frequencies.
A feature of the second arrangement is that it has a printed ring shaped conductor which is coupled to two signal ports at points which are approximately 90xc2x0 apart on the conductor ring. The ring conductor is coupled to a printed cross-slot conductor pattern which, during operation develops across the respective slots, two electromagnetic fields at two mutually orthogonal polarisations. The use of a cross-slot pattern to match a patch antenna has previously been proposed, for instance by Edimo et al in Electronics Letters, 10 Sep. 1992, Vol. 28, No. 19, but there was no suggestion that a circular ring-shaped or other shaped loop conductor should be used to provide the coupling with the cross slots. Since the two signal input ports excite orthogonal radiation modes, there is little or negligible interaction between them.
In particular arrangements to be described below, by way of example, in illustration of the invention, crossed slot fields excite xe2x80x98fringingxe2x80x99 electromagnetic fields around the edge of a metal patch, from which when the antenna is transmitting they radiate as two separate, but substantially coincident, conically shaped propagation patterns. The patch is not essential to the operation of the embodiments, but it results in the provision of more concentrated beams, i.e. beams having a narrower angle of propagation than they would otherwise have. Other parasitic elements may be used to provide other shapes of propagation pattern. On the other hand it is possible to employ embodiments having no parasitic element, such as a patch.
It is also possible to employ a reflector plate in order to confine the beams to one general direction of propagation. On the other hand, should propagation in two opposite directions be required, or not be objectionable, it is possible to omit a reflector plate.